[CMake] Revisit PR23539: allow to override SANITIZER_MIN_OSX_VERSION at configure time.

Some users may need to configure Clang/compiler-rt with
-DMACOSX_DEPLOYMENT_TARGET=10.6, as they expect Clang to produce working
(non-sanitized) binaries on 10.6. In this case they would need to
additionally provide -DSANITIZER_MIN_OSX_VERSION=10.7 to mark
that sanitizer runtimes may only be used on 10.7+ (where they are
supported).

llvm-svn: 242363